[dpdk-dev] mbuf changes

[Bruce Richardson]

On Mon, Oct 24, 2016 at 04:11:33PM +0000, Wiles, Keith wrote:
> 
> > On Oct 24, 2016, at 10:49 AM, Morten Brørup <mb at smartsharesystems.com> wrote:
> > 
> > First of all: Thanks for a great DPDK Userspace 2016!
> > 
> > 
> > 
> > Continuing the Userspace discussion about Olivier Matz’s proposed mbuf changes...

Thanks for keeping the discussion going!
> > 
> > 
> > 
> > 1.
> > 
> > Stephen Hemminger had a noteworthy general comment about keeping metadata for the NIC in the appropriate section of the mbuf: Metadata generated by the NIC’s RX handler belongs in the first cache line, and metadata required by the NIC’s TX handler belongs in the second cache line. This also means that touching the second cache line on ingress should be avoided if possible; and Bruce Richardson mentioned that for this reason m->next was zeroed on free().
> > 
Thinking about it, I suspect there are more fields we can reset on free
to save time on alloc. Refcnt, as discussed below is one of them, but so
too could be the nb_segs field and possibly others.

> > 
> > 
> > 2.
> > 
> > There seemed to be consensus that the size of m->refcnt should match the size of m->port because a packet could be duplicated on all physical ports for L3 multicast and L2 flooding.
> > 
> > Furthermore, although a single physical machine (i.e. a single server) with 255 physical ports probably doesn’t exist, it might contain more than 255 virtual machines with a virtual port each, so it makes sense extending these mbuf fields from 8 to 16 bits.
> 
> I thought we also talked about removing the m->port from the mbuf as it is not really needed.
> 
Yes, this was mentioned, and also the option of moving the port value to
the second cacheline, but it appears that NXP are using the port value
in their NIC drivers for passing in metadata, so we'd need their
agreement on any move (or removal).

> > 
> > 
> > 
> > 3.
> > 
> > Someone (Bruce Richardson?) suggested moving m->refcnt and m->port to the second cache line, which then generated questions from the audience about the real life purpose of m->port, and if m->port could be removed from the mbuf structure.
> > 
> > 
> > 
> > 4.
> > 
> > I suggested using offset -1 for m->refcnt, so m->refcnt becomes 0 on first allocation. This is based on the assumption that other mbuf fields must be zeroed at alloc()/free() anyway, so zeroing m->refcnt is cheaper than setting it to 1.
> > 
> > Furthermore (regardless of m->refcnt offset), I suggested that it is not required to modify m->refcnt when allocating and freeing the mbuf, thus saving one write operation on both alloc() and free(). However, this assumes that m->refcnt debugging, e.g. underrun detection, is not required.

I don't think it really matters what sentinal value is used for the
refcnt because it can't be blindly assigned on free like other fields.
However, I think 0 as first reference value becomes more awkward
than 1, because we need to deal with underflow. Consider the situation
where we have two references to the mbuf, so refcnt is 1, and both are
freed at the same time. Since the refcnt is not-zero, then both cores
will do an atomic decrement simultaneously giving a refcnt of -1. We can
then set this back to zero before freeing, however, I'd still prefer to
have refcnt be an accurate value so that it always stays positive, and
we can still set it to "one" on free to avoid having to set on alloc.

Also, if we set refcnt on free rather than alloc, it does set itself up
as a good candidate for moving to the second cacheline. Fast-path
processing does not normally update the value.

> > 
> > 
> > 
> > 5.
> > 
> > And here’s something new to think about:
> > 
> > m->next already reveals if there are more segments to a packet. Which purpose does m->nb_segs serve that is not already covered by m->next?

It is duplicate info, but nb_segs can be used to check the validity of
the next pointer without having to read the second mbuf cacheline.

Whether it's worth having is something I'm happy enough to discuss,
though.

One other point I'll mention is that we need to have a discussion on
how/where to add in a timestamp value into the mbuf. Personally, I think
it can be in a union with the sequence number value, but I also suspect
that 32-bits of a timestamp is not going to be enough for many.

Thoughts?

/Bruce